Semiconductor processing techniques have been making remarkable progress, and processing techniques with precision of the order of 100 nm are about to become commercially practical. As semiconductor devices decrease in size, switching speeds increase while power consumption decreases. Thus, downsizing of semiconductor devices is essential for the production of high-performance LSIs. Up to now, the packing density of semiconductor devices has linearly increased with each passing year. However, it is a matter of time that the processing precision reaches the limit of photolithography in conventional use, so that the development of an alternative processing process is an urgent necessity.
As a processing process exceeding the limit of the photolithography in conventional use, more attention is being paid to a process for causing spontaneous production of a microstructure utilizing properties of a material or the like based on so-called self-organization. The microstructure formed by self-organization exhibits a wide variety of forms such as a layer form, a fiber form, a columnar form, a spherical form, and a porous form, and potential applications are proposed for the respective forms. Above all, porous thin films formed on a substrate and columnar structured materials particularly have a wide field of industrial applications, being considered as the most promising applications.
Among the most noteworthy applications of the porous thin films are alumina nanoholes formed by anodizing aluminum. The alumina nanoholes are obtained by anodizing an aluminum thin film under given conditions as microholes formed perpendicularly to the surface due to electrostatic focus.
As other significant materials, there are thin films of mesoporous materials produced by a sol-gel process or the like with a surfactant aggregate being used as a mold. Those materials are obtained by producing materials having a regular microhole structure by such a simple method as dip coating, and are provided with microholes arranged in parallel to substrates.
Meanwhile, as to the columnar structured materials, there are many materials being under study. The production methods for the columnar structured material are broadly divided into two methods. One is a method (first method) of producing a structured material having a columnar structure directly onto a substrate. The other is a method (second method) of forming an object material within microholes formed in a porous material and then removing the porous material.
First, description is made of the first method. As the method of forming a columnar structured material directly onto a substrate, there are a method of performing deposition from a liquid phase and a method of performing growth from a vapor phase. The method of performing deposition from a liquid phase is used to form the columnar structured material of ZnO, TiO2, or the like with a relatively low aspect ratio. As the method of performing growth from a vapor phase, which is used more generally, there are a method of performing vapor deposition directly on a raw material, a method of performing growth by catalytic reaction using a Vapor-Liquid-Solid mechanism, a method using Chemical Vapor Deposition (CVD), and the like. For example, it has been reported that a catalyst such as gold can be used to grow a needle crystal of zinc oxide on a substrate. Those methods are used to obtain the columnar structured materials of a metal, a semiconductor, or the like with a high aspect ratio on a substrate.
Next, description is made of the second method. As a porous material used in this method, the microholes preferably have a linear form without having a branch. For example, the above-mentioned alumina nanoholes and mesoporous materials can be used. As to the use of the alumina nanoholes, for example, by using a conductive material as a base substrate under a nanohole film, the object material is introduced into the microholes by such a method as electrodeposition, and alumina is finally removed to produce the columnar structured material. When such a method as electrodeposition is used, it is also possible to produce a columnar structured material different in compositions from the middle of the process. In the case of the mesoporous materials, the columnar structure can be formed by a method of adsorbing a precursor of the object material and then using chemical treatment for an object composition, a CVD method, or the like. In the case of using the mesoporous materials, a longitudinal direction of the columnar structure becomes parallel to the substrate. Adjacent columns are joined by a microfine wire. There is an example having description of platinum as to the production of the columnar structured material using the mesoporous materials.
(Prior Art Relating to Electrode)
As a detection method for molecules, ions, and the like dissolved in a solution, an electrochemical measuring method for performing a measurement of a current value in accordance with the transfer of electrode electrons is used, and is currently being put into application in various fields.
In particular, in a measurement for micro detection or the like, it is important to obtain a current response within a given observation area with high precision and high sensitivity. In order to achieve this object, attention is being given to enlargement of a surface area of an electrode and regulation of a structured material.
In recent years, there is proposed a production method for an electrode having a columnar structured material to which photolithography is applied. Known examples of this method include: a lift-off process in which a resist is applied to a substrate, a photomask having an electrode pattern is superposed thereon, exposure and development are performed, a metal thin film is then formed by vapor deposition or the like, and the resist is peeled off to obtain a microelectrode on the substrate; and an etching process in which a metal thin film is produced on an insulating substrate, a resist is then subjected to application, exposure, and development in this order, and a residual resist is further used as a mask to subject the metal thin film in an exposed portion to etching to obtain an electrode pattern. With the above method, a large number of microelectrodes having an arbitrary form and a predetermined interelectrode distance can be produced on a substrate with high reproducibility.
However, the photolithography in current use has a limit of precision of the order of approximately 100 nm. Therefore, as a processing process exceeding the limit of the photolithography, more attention is being paid to a process for causing spontaneous production of a microstructure utilizing properties of a material or the like based on so-called self-organization. A method of producing an electrode having a columnar structure of the order of 100 nm or less by using the self-organization is broadly divided into two methods. One is the above-mentioned first method of forming a columnar structured material directly onto a substrate, and the other is the above-mentioned second method of forming an object material within microholes formed in a porous material and then removing the porous material. Description is further made of reported examples of use of the second method that has been already described to produce an electrode having a columnar structured material for detection with high precision and high sensitivity. For example, it is reported in Bull. Chem. Soc. Jpn., 66, 305(1993) that alumina nanoholes are used for a mold to deposit Ni by electrolysis, and in Japanese Patent Application Laid-Open No. 2000-001392 that alumina nanoholes are used for a mask to deposit a metal, and the metal is used as a catalyst to grow a columnar structured material and obtain an electrode.
However, the above-mentioned respective methods of producing a columnar structured material have problems as described below.
First, in the method of forming a columnar structured material directly from a liquid phase, applicable materials are limited. For example, in the case of using electrodeposition, wide-range materials such as metals are formed mainly into a form of a continuous film. Further, even in the case where a material can be formed into a columnar structure, it is difficult to orient its direction so as to be completely perpendicular to a substrate. Even a single column of the columnar structured material often has its proximal end and its distal end varied in diameter. In the method of forming a columnar structured material directly from a vapor phase, a process performed at high temperature becomes necessary in many cases, so that only such a substrate that can withstand a formation temperature for the object columnar structured material can be applied. Also, in the case of using catalytic reaction, a specific material of a noble metal or the like needs to be formed on a surface of the substrate, causing problems with a configuration of the structured material and production cost. In this case as well, a single column of the columnar structured material often has its proximal end and its distal end varied in diameter. Meanwhile, in the method of producing a columnar structured material by introducing a material into alumina nanoholes, it is substantially impossible in actuality to set the diameter of a microhole among the alumina nanoholes to 10 nm or less. Thus, it is difficult to control the diameter of the formable columnar structured material to 10 nm or less, and a technique for producing a columnar structured material having a further smaller diameter is desired. In many cases, it is when the size becomes less than 10 nm that the material is released from bulk properties and develops specific properties such as a quantum size effect. Therefore, it has been a significant problem to produce a columnar structured material having a diameter of less than 10 nm.
Further, in the method of producing a columnar structured material by introducing a material into a mesoporous material, obtained microholes are sufficiently small in diameter, but an orientation of the microholes is parallel to the substrate. Thus, it is difficult to apply a process with ease and high reliability such as electrodeposition for introducing the material into the microholes.